Automatic gain control circuits



Dec, 17, 1%35. F. M. G. MURPHY AUTOMATIC GAIN CONTROL CIRCUITS Filed May 28, 1954 3 Sheets-Sheet l RECT/F/E? OUTPUT 1/0 NAQQQ INVENTOR FRANC/S MG- MOP/ H) 'ATTo RN EY Dec. 17, 1935. F, M G, MURPH? 2,@25,019

AUTOMATIC GAIN CONTROL CIRCUITS Filed May 28, 1954 5 Sheets-Sheet 2 M INVENTOR FAAA/C/S M. G. MURPHY ALI'TORNEY Dec. 17, 1935. MURPHY 2,025,019

' AUTOMATIC GAIN CONTROL CIRCUITS Filed May 28, 1934 3 Sheets-Sheet 5 J5 4'4 4'3 4'2 #1 -/'0 s 43 -'7 -'5 5 r 3 '2 0 E 5/45 was NVENTOR 1994/1 05 M6 MURPHY IKZW ATTORNEY time.

Patented Dec. 17, 1935 UNITED STATES PATENT OFFICE AUTOMATIC GAIN CONTROL CIRCUITS Application May 28, 1934, Serial No. 727,921 In Great Britain July 12, 1933 4 Claims.

This invention relates to thermionic valve circuit arrangements and more particularly to such arrangements for use for control purposes in telephone systems.

It is commonly required to provide a thermionic valve circuit arrangement which shall rectify speech or similar alternating current input to give a rectified output in dependence upon said alternating input and for many purposes it may be desired that the ratio between the rectified output and the alternating input shall change fairly sharply and suddenly when the said alternating input rises above or falls below a predetermined level of amplitude.

A typical example wherein the above requirements arise is that of a two-way radio or like telephone system having so-called ,blocking arrangements. The specification accompanying copending application Serial No. 690,320, filed September 21st, 1933 describes a two-way telephone system embodying so-called blocking devices, these blocking devices operating in such manner as to prevent the transmitting and receiving channels being in operation at the same In the embodiments described in the copending specification just referred to, the blocking action is obtained by control of grid bias voltages applied to certain valves in the system. In Fig. 4 of the copending specification is illustrated a complete two-way terminal equipment for a radio telephone system and at Z in the said Figure 4 is shown diagrammatically an arrangement in accordance with the present invention for giving a control action in dependence upon signal strength. As set forth in the specification Serial No. 690,320 the apparatus at Z in Fig. 4 accomplishes differentiation as between signals above and signals below a predetermined level (there being a rectified output from said apparatus at Z for the stronger input levels and substantially none for weaker levels) and, since this rectified output is utilized (as described in specification Serial No. 690,320) as a control potential in connection with blocking, differentiation as regards control is obtained as between received speech signals and noise since the level of the latter is below that of the former.

- The present invention has for its object to provide an improved apparatus whereby an output potential, which may be used for control appa-' ratus, may be obtainedin dependence upon input alternating potentials the interdependence of output and input potentials being such that a sharp change in the ratio therebetween occurs when a predetermined level of input amplitude is passed.

According to this invention a thermionic ,valve circuit arrangement comprises a thermionic valve to whose input circuit alternating current input 5 is applied, and one or more rectifiers energized in dependence upon the output current from said valve, means being provided for utilizing the whole or part of the voltage set up by the passage of said rectified current through an im- 10' pedance as the whole or part of the grid bias potential applied to said valve, the amount of rectified output current potential employed as grid bias being such that there is a sharp change in output alternating current from the valve 15 when the alternating input rises above or falls below a predetermined value of amplitude.

In the drawings:

Fig. 1 shows a. graphical analysis of the invention,

Fig. 2 shows one embodiment of the invention,

Fig. 3 illustrates a modification of the invention,

Figs. 4, 5 and 6 respectively show different characteristics of the circuits illustrated in Figs. 25' 2 and 3.

In order that the invention may be better understood it will first be described from the theoretical aspect before an actual practical embodiment is described.

Consider the case of a thermionic valve having alternating current potentials applied between the grid and cathode thereof and having in its plate circuit the primary of a transformer whose secondary feeds a rectifier. Now, of course,

the current through the rectifier may be regarded as proportionate to the mean value of the plate circuit alternating current (neglecting the effect of the transformer or assuming no transformation efiect) If, for such an arrangement, 40 different alternating amplitudes and different values of grid bias be applied to the triode there will be obtained a family of curves which relates rectifier output voltage to these two variables.

Such a family of curves is typified by the curved graphs of Fig. 1, and in the said figure the abscissa values are values of grid bias and the ordinate values are values of voltage set up by passing the rectified current through a fixed resistance.

Suppose now that the resistance through which the rectified output current is passed or a part thereof, is included also in the grid circuit of the valve so that the total grid bias on the valve is the algebraic sum of the potential due to the passage of this rectified current through said m resistance and the potential of any other bias source provided. For example, suppose that the valve grid circuit includes a grid bias battery giving l volts bias. Then, in the absence of any alternating input, the bias on the valve would be volts, but (assuming the whole of the rectified output current voltage to be fed back as grid bias in opposition to the bias battery) the total effective grid bias will become zero when 10 the alternating input of the valve is such that the rectified output voltage is 10 volts. This is illustrated graphically in Fig. 1 by means of the straight line AA which is drawn from a point on the abscissa line corresponding to the fixed grid bias, (i. e. bias due to a grid bias battery or the like) and at an angle to the abscissa line such that the co-tangent of this angle is equal to that fraction of the rectified output voltage which is applied as grid bias. In other words, if all the rectified output voltage is fed back as grid bias in opposition to the grid bias battery the angle AAO will be such as to have a co-tangent equal to 1; if one half the rectified output voltage is fed back the angle A'AG will be such as to have a (IO-tangent equal to 0.5 and soon.

Now it will be obvious that as various levels of alternating input are applied to the grid of the valve the total effective grid bias will be given by the abscissa values appropriate to the intersec- 3 'tion of the relevant curve with the line AA (assuming the steady bias has a value of A) so long of course as the curve only intersects the line AA at one point. If, however, as is the case with the curve marked I in Fig. 1 said curve is 5- -tangential to AA and later intersects it, any small increment of level or decrease of bias will produce a greater decrease of total effective bias than is required to maintain operation on the line AA. The state is therefore unstable; the rectifier output will increase and the total effective bias decrease until the curve again intersects the line AA.

In practice the adjustments may be made such that the effect obtained is that, as the input a1- 5 ternating level is gradually increased the output gradually increases until a second point is reached when the output suddenly increases considerably and with gradually decreasing input amplitudes a second point is reached when the output decreases suddenly. This second point is that given by the curve 2 of Fig. 1 which intersects the line AA', and then becomes tangential to it. It follows, therefore, that in general there is a hysteresis effect, i. e. the results obtained for increasing values of input are not identical quantitatively with those obtained for decreasing values of input, but by carefully choosing the value of the grid bias, and the slope of the line AA it is possible for any particular suitable type of 0- valve, so to arrange matters that the hysteresiseffect is very small, or at any rate not serious from a practical point of view. It will also have been noted from the preceding description that the rectified output increases gradually before 5 reaching instability, but, by suitable choice of the bias voltage and of the fraction of rectified output voltage fed back to the grid as grid bias this difficulty can be reduced so that it does not remain serious practically.

7 One arrangement in accordance with the invention is shown diagrammatically in Fig. 2, wherein input alternating potentials are applied through the usual input transformer T between grid and cathode of a triode V whose plate cir- 757 cuit includes an output transformer OT having one primary P1 and two secondaries S1 S2. The secondary S1 is connected across a diode rectifier D, the rectified current from which sets up voltage in a resistance or other suitable impedance R.

This voltage constitutes the useful output or 5 control voltage and may be utilized for some desired control purpose, for example as illustrated in Fig. 4 of the specification Serial No. 690,320. The second secondary S2 feeds voltage to a rectifier unit consisting of a plurality of metal 10 rectifiers connected as illustrated to act as a full wave rectifier. This rectifier has a load circuit consisting of a condenser C shunted by a resistance LR and an adjustable tapping point A]? upon the resistance LR is connected to the lower 15 end of the secondary of the transformer T as shown. The negative terminal of the grid bias battery for the valve V is connected at -GB1 so that the total grid bias voltage upon the said valve V at any time will be the algebraic sum of 20 the voltage due to the bias battery (not shown) and the voltage set up across that portion of the resistance LR which is in the grid circuit of the said valve V. -GB denotes the negative terminal of the bias battery for the valve shown in Fig. 2 5

as following the resistance R.

With the arrangement illustrated it will be appreciated that adjustment of the potentiometer constituted by the tapping point AP upon the resistance LR in effect provides variation of the g slope of the line AA of Fig. 1. It will further be noted that substantial negative bias is applied to the diode rectifier D. This negative bias tends to meet the difliculty above referred to, namely that the rectifier output increases gradually be- 5' fore reaching the point of instability.

The purpose of the resistance L connected in the grid of the valve V is to prevent adverse effects due to overloading i. e. the application of excessively high alternating current amplitudes 40" to the primary of the transformer T.

A preferred circuit arrangement in accordance with the invention, and which differs from the arrangement shown in'Fig. 2 in certain particulars in shown diagrammatically in Fig. 3. A 45 detailed description of this figure is considered unnecessary, but it will be noted that it difiers from the arrangement illustrated in Fig. 2 in that it includes a preceding amplification stage comprising a valve V0 and input transformer Tov 5 and an intervalve coupling transformer T corresponding to the input transformer T in Fig. 2.

The grid circuit of the valve V includes as before a resistance L, and an output transformer OT is connected in the anode circuit of valve V. 55

Said transformer has two secondary windings; 5;, connected across the diode D, and S2 feeding a metal rectifier R which it will be seen is of the voltage-doubler type instead of the bridge type. The voltage doubler type of rectifier is 6(1 found to be preferable on account of the lower back-leakage as compared with the bridge type. It will further be noted that a resistance r has been included in series between the metal rectifier and the associated winding S2. It will be noted that in the arrangements illus-- trated in Fig. 2 and in Fig. 3, separate rectifier circuits are provided for giving the useful or control output D. C. voltage and forgiving the rectified or D. C. voltage for utilization as grid bias- This is a very convenient arrangement from the point of view of simplicity and ease of adjustment. Further curves of the type shown in Fig. 1 arev given in the accompanying Fig. 4, and show the effect of employing valves of different character- 7 1 istics for the valve V with a fixed value of rectifier load resistance LR of 100,000 ohms, the diode D being disconnected. The chain line curves were obtained with a valve of the type known under the trade designation NIL-4; the full line full line curves with a valve of the type known under the trade designation MHL- i; and the dotted line curves with a valve of the type known under the trade designation MH-4. The NIL-4 type tube is one which has the following data: Filament volts-4.0; filament current1.0 ampere; anode volts max-200; amplification factor-12; impedance-2860 ohms; mutual conductance-4.2 MA/V. The MEL-4 type tube differs from the lVl'L-4 type in that it has an amplification factor of 20; an impedance of 8000 ohms; and a mutual conductance of 2.5 MA/V. The MEI-4 type tube differs from the ML-4 type tube in that it has an amplification factor of 40; an impedance of 11,100 ohms; and a mutual conductance of 3.6 MA/V. The numbers adjacent the curves indicate the input level in decibels referred to 1 milliwatt as parameter, the input being of 1,000 cycles frequency.

A further family of curves similar to those of Fig. 4, but showing the effect of employing a different rectifier load resistance, namely, one of 20,000 ohms, is given in the accompanying Fig. 5. These curves were obtained with an MH-4 valve.

1 The curves shown in Fig. 6 were obtained with a valve V of the type ML-4 and with arectifier load resistance of 20,000 ohms, the diode D in this case being connected in circuit. All these sets of curves were obtained with a circuit arrangement comprising a two-element rectifier and a preliminary amplification stage generally as illustrated in Fig. 3. The inclusion, as shown in Fig. 3, of the resistance 1- between the rectifier R and the associated transformer winding S2 tends to minimize the hysteresis effect hereinbefore referred to by causing the static characteristic curves (e. g. those shown in Fig. to exhibit a more rapid decrease of slope in the neighborhood of their maxima.

What is claimed is:-

1. In combination with a signal amplifier, a load circuit, means for rectifying amplified signals for impression on the load circuit, a second rectifying means independent of the first means having an input circuit coupled to the amplifier, said second means including an output impedance for developing thereacross a rectified signal Voltage, connections for applying a predetermined fraction of said voltage to the amplifier input circuit whereby the gain of the amplifier is varied in dependence upon the amplitude of the signals transmitted to the second rectifier means, said fraction being chosen to produce a sharp change in alternating current output from the amplifier when the signal input to the amplifier rises above, or falls below, a predetermined value of amplitude,

means for applying a bias voltage of fixed value to the amplifier input circuit, the bias voltage being of opposite sign from the said rectified voltage.

2. In combination with a signal amplifier, a load circuit, means for rectifying amplified sig- 5 nals for impression on the load circuit, a second rectifying means independent of the first means having an input circuit coupled to the amplifier,

said second means including an output impedance for developing thereacross a. rectified signal voltage, connections for applying a predetermined fraction of said voltage to the amplifier input circuit whereby the gain of the amplifier is varied in dependence upon the amplitude of the signals transmitted to the second rectifier means, means for applying a bias voltage of fixed value to the amplifier input circuit, the bias voltage being of opposite sign from the said rectified voltage, said second rectifier means including an impedance in its input circuit for substantially minimizing hysteresis effect during rectification.

3. In combination with a signal amplifier, a load circuit, means for rectifying amplified signals for impression on the load circuit, a second rectifying means independent of the first means having an input circut coupled to the amplifier, said second means including an output impedance for developing thereacross a rectified signal voltage, connections for applying a predetermined fraction of said voltage to the amplifier input circuit whereby the gain of the amplifier is varied in dependence upon the amplitude of the signals transmitted to the second rectifier means, means for applying a bias voltage of fixed value to the amplifier input circuit, the bias voltage being of opposite sign from the said rectified voltage, said second rectifying means being of the voltage doubling type, and means for biasing the first rectifier means to prevent the first rectifier output from gradually increasing before reaching a point of instability.

4. In combination with a signal amplifier, a load circuit, means for rectifying amplified signals for impression on the load circuit, a second rectifying means independent of the first means having an input circuit coupled to the amplifier, said second means including an output impedance for developing thereacross a rectified signal voltage, connections for applying a predetermined fraction of said voltage to the amplifier input circuit whereby the gain of the amplifier is varied in dependence upon the amplitude of the signals transmitted to the second rectifier means, means for applying a bias voltage of fixed value to the amplifier input circuit, the bias voltage being of opposite sign fro-m the said rectified voltage, said load circuit including an amplifier, and a common means for biasing the grid of the last amplifier and the said first rectifier means.

FRANCIS MONEY GRAHAM MURPHY. 

